Affinity labeling reagents that have been developed for the estrogen receptor will be used in conjunction with high-resolution protein fractionation techniques, to determine the physico-chemical characteristics of receptor molecules. Estrogen receptors from different target tissues of the same species will be compared to see whether any differences in properties are evident. If differences appear, they will form the basis of a molecular rationale that will be utilized to design more selective estrogen pharmaceutical agents. The differences between receptor-antiestrogen and receptor-estrogen complexes that we have already observed, will be further documented, and these differences will be used to devise an in vitro assay of the antagonist activity of estrogen pharmaceuticals. Receptor characterization studies will be extended to breast tumor samples to try to place on both a qualitative and a quantitative basis the relationship between estrogen receptor and response to hormonal therapy. New approaches to identifying and isolating nuclear acceptor sites based on a combination of affinity labeling and crosslinking reactions will be investigated, and photoaffinity labeling reagents will be used to study the cellular dynamics of estrogen-receptor interactions. New affinity labeling reagents that have higher receptor binding affinity (but lower non-specific binding) and high reaction efficiencies will be synthesized. A series of novel chemically- and photochemically-reactive functional groups, as well as the nuclear decay of iodine-131 labeled aromatic compounds, will be evaluated as covalent attaching functions. Materials for affinity chromatography of receptors will be prepared from photoisomerizable or photofragmentable ligands. The reagents and techniques for receptor characterization that will be developed and utilized in this project should permit advances both in the understanding of the molecular details of steroid hormone action and in the preparation of steroid hormone pharmaceuticals that are safer and more effective.